public static int commonChild(String s1, String s2) {
    //watch lecture at 
    //https://www.youtube.com/watch?v=jHGgXV27qtk
    //to understand how to solve 
    //this problem using dynamic programming
    char c1;
    char c2;
    int max;
    
    int n = s1.length();
    int[][] memo = new int[n + 1][n + 1];
    
    for (int i = 0; i < n + 1; i++) {
        for (int j = 0; j < n + 1; j ++) {
            if (i == 0 || j == 0) {
                memo[i][j] = 0;
            }
            else {
                c1 = s1.charAt(i - 1);
                c2 = s2.charAt(j - 1);
                
                if (c1 != c2) {
                    max = Math.max(memo[i - 1][j], memo[i][j - 1]);
                    memo[i][j] = max;
                }
                else {
                    memo[i][j] = memo[i - 1][j - 1] + 1;
                }
            }
        }
    }
    return memo[n][n];
    /* GETTING THE LONGEST COMMON STRING
    int i = n;
    int j = n;
    
    LinkedList<Character> commonString = new LinkedList<>();
    
    while (memo[i][i] != 0) {
        char c1 = s1.charAt(i - 1);
        char c2 = s2.charAt(j - 1);
        
        if (c1 == c2) {
            commonString.addFirst(c1);
            i--;
            j--;
        }
        else if (memo[i][j - 1] >= memo[i - 1][j]) {
            j--;
        } 
        else {
            i--;
        }
    }
    
    StringBuilder builder = new StringBuilder();
    for (Character c : commonString) {
        builder.appends(c);
    }
    
    String common = builder.toString();
    */